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Abstract:

A system, including: an implantable neural stimulator including
electrodes, at least one antenna and an electrode interface; a
radio-frequency (RF) pulse generator module comprising an antenna module
configured to send an input signal to the antenna in the implantable
neural stimulator through electrical radiative coupling, the input signal
containing electrical energy and polarity assignment information that
designates polarity assignments of the electrodes in the implantable
neural stimulator; and wherein the implantable neural stimulator is
configured to: control the electrode interface such that the electrodes
have the polarity assignments designated by the polarity assignment
information, create one or more electrical pulses suitable for modulation
of neural tissue using the electrical energy contained in the input
signal, and supply the electrical pulses to the electrodes through the
electrode interface such that the electrodes apply the electrical pulses
to the neural tissue with the polarity assignments designated by the
polarity assignment information.

Claims:

1. A system for modulating neural tissue in a patient comprising: an
implantable neural stimulator comprising one or more electrodes, at least
one antenna and an electrode interface; a radio-frequency (RF) pulse
generator module comprising an antenna module configured to send an input
signal to the antenna in the implantable neural stimulator through
electrical radiative coupling, the input signal containing electrical
energy and polarity assignment information that designates polarity
assignments of the electrodes in the implantable neural stimulator; and
wherein the implantable neural stimulator is configured to: control the
electrode interface such that the electrodes have the polarity
assignments designated by the polarity assignment information, create one
or more electrical pulses suitable for modulation of neural tissue using
the electrical energy contained in the input signal, and supply the
electrical pulses to the electrodes through the electrode interface such
that the electrodes apply the electrical pulses to the neural tissue with
the polarity assignments designated by the polarity assignment
information.

2. The system of claim 1 wherein the implantable neural stimulator is
configured to transmit a stimulus feedback signal indicating one or more
parameters of the electrical pulses to the antenna module of the RF pulse
generator module.

3. The system of claim 2 wherein the RF pulse generator module comprises
one or more circuits coupled to the antenna module and configured to
receive the stimulus feedback signal adjust parameters of the input
signal based on the stimulus feedback signal.

4. The system of claim 1 wherein the antenna module is configured to
transmit to the implantable neural stimulator a first input signal
containing electrical energy and a second input signal containing the
polarity assignment information, wherein the first input signal has a
different carrier frequency than the second input signal.

5. The system of claim 2 wherein the antenna module comprises a first
antenna configured to operate at a first frequency to transmit the first
input signal and a second antenna configured to operate at a second
frequency to receive the stimulus feedback signal from the antennas of
the implantable neural stimulator, wherein the second frequency of the
second antenna is higher than a resonant frequency of the first antenna.

6. The system of claim 5 wherein the second frequency of the second
antenna is a second harmonic of the resonant frequency of the first
antenna.

7. The system of claim 5 wherein the second frequency and the resonant
frequency are in a range from 300 MHz to 6 GHz.

8. The system of claim 1 wherein the antenna of the implantable neural
stimulator is between about 0.1 mm and 7 cm in length and between about
0.1 mm to 3 mm in width.

9. The system of claim 1 wherein the antenna of the implantable neural
stimulator is a dipole antenna.

10. The system of claim 1 wherein: the polarities designated by the
polarity assignment information include a negative polarity, a positive
polarity, or a neutral polarity; the electrical pulses include a cathodic
portion and an anodic portion; and the electrode interface comprises a
polarity routing switch network that includes a first input that receives
the cathodic portion of the electrical pulses and a second input that
receives the anodic portion of the electrical pulses, the polarity
routing switch network configured to route the cathodic portion to
electrodes with a negative polarity, route the anodic portion to
electrodes with a positive polarity, and disconnect electrodes with a
neutral polarity from the electrical pulses.

11. The system of claim 10 wherein the implantable neural stimulator
comprises one or more circuits having a register with an output coupled
to a selection input of the polarity routing switch network, wherein the
register is configured to store the polarity assignment information and
send the stored polarity assignment information from the register output
to the selection input of the polarity routing switch network to control
the polarity routing switch network to route the cathodic portion to
electrodes with a negative polarity, route the anodic portion to
electrodes with a positive polarity, and disconnect electrodes with a
neutral polarity from the electrical pulses.

12. The system of claim 11 wherein the one or more circuits include a
power-on reset circuit and a capacitor, wherein the capacitor stores a
charge using a portion of the electrical energy contained in the input
signal, and wherein the capacitor is configured to energize the power-on
reset circuit to reset the register contents when the implanted neural
stimulator loses power.

13. The system of claim 12 wherein the parameters include a current and a
voltage of the electrical pulses, and the implantable neural stimulator
comprises a current sensor configured to sense an amount of current in
the electrical pulses and a voltage sensor configured to sense a voltage
in the electrical pulses.

14. The system of claim 13 wherein the current sensor and the voltage
sensor are coupled to a resistor placed in serial connection with an
input of the polarity routing switch network that receives an anodic
portion of the electrical pulses.

15. The system of claim 14 wherein the current sensor and the voltage
sensor are coupled to an analog controlled carrier modulator, the analog
controlled carrier modulator being configured to communicate the sensed
current and voltage to the antenna module.

16. The system of claim 1 wherein the implantable neural stimulator
comprises a rectifying circuit coupled to a RC timer, wherein the
rectifying circuit is configured to rectify the input signal received by
the antenna module to generate the electrical pulses and the RC-timer is
configured to shape the electrical pulses.

17. The system of claim 16 wherein the rectifying circuit comprises at
least one full wave bridge rectifier, wherein the full wave bridge
rectifier comprises a plurality of diodes, each diode being less than 100
micrometers in length.

Description:

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application is a divisional application of U.S. patent
application Ser. No. 13/562,221, filed Jul. 30, 2012, which claims
benefit of U.S. provisional Patent Application 61/513,397, filed Jul. 29,
2011, and is a continuation-in-part of PCT Application PCT/US2012/023029,
filed Jan. 27, 2012, which claims benefit of U.S. provisional Patent
Application 61/437,561, filed Jan. 28, 2011, all of which are hereby
incorporated by reference in their entirety.

TECHNICAL FIELD

[0002] This description is related to implanted neural stimulators.

BACKGROUND

[0003] Neural modulation of neural tissue in the body by electrical
stimulation has become an important type of therapy for chronic disabling
conditions, such as chronic pain, problems of movement initiation and
control, involuntary movements, dystonia, urinary and fecal incontinence,
sexual difficulties, vascular insufficiency, heart arrhythmia and more.
Electrical stimulation of the spinal column and nerve bundles leaving the
spinal cord was the first approved neural modulation therapy and been
used commercially since the 1970s. Implanted electrodes are used to pass
pulsatile electrical currents of controllable frequency, pulse width and
amplitudes. Two or more electrodes are in contact with neural elements,
chiefly axons, and can selectively activate varying diameters of axons,
with positive therapeutic benefits. A variety of therapeutic intra-body
electrical stimulation techniques are utilized to treat neuropathic
conditions that utilize an implanted neural stimulator in the spinal
column or surrounding areas, including the dorsal horn, dorsal root
ganglia, dorsal roots, dorsal column fibers and peripheral nerve bundles
leaving the dorsal column or brain, such as vagus-, occipital-,
trigeminal, hypoglossal-, sacral-, and coccygeal nerves.

SUMMARY

[0004] In one aspect, an implantable neural stimulator includes one or
more electrodes, at least one antenna, and one or more circuits connected
to at least one antenna. The one or more electrodes are configured to
apply one or more electrical pulses to excitable tissue. The antenna is
configured to receive one or more input signals containing polarity
assignment information and electrical energy, with the polarity
assignment information designating polarities for each of the electrodes.
The one or more circuits are configured to control an electrode interface
such that the electrodes have the polarities designated by the polarity
assignment information; create one or more electrical pulses using the
electrical energy contained in the input signal; and supply the one or
more electrical pulses to the one or more electrodes through the
electrode interface such that the one or more electrodes apply the one or
more electrical pulses to excitable tissue according to the polarities
designated by the polarity assignment information.

[0005] Implementations of this and other aspects may include the following
features. The polarities designated by the polarity assignment
information may include a negative polarity, a positive polarity, or a
neutral polarity. The electrical pulses include a cathodic portion and an
anodic portion. The electrode interface may include a polarity routing
switch network. The polarity routing switch network may include a first
input that receives the cathodic portion of the electrical pulses and a
second input that receives the anodic portion of the electrical pulses.
The polarity routing switch network may be configured to route the
cathodic portion to electrodes with a negative polarity, route the anodic
portion to electrodes with a positive polarity, and disconnect electrodes
with a neutral polarity from the electrical pulses.

[0006] The one or more circuits may include a register with an output
coupled to a selection input of the polarity routing switch network. The
register may be configured to store the polarity assignment information
and send the stored polarity assignment information from the register
output to the selection input of the polarity routing switch network to
control the polarity routing switch network to route the cathodic portion
to electrodes with a negative polarity, route the anodic portion to
electrodes with a positive polarity, and disconnect electrodes with a
neutral polarity from the electrical pulses.

[0007] The one or more circuits include a power-on reset circuit and a
capacitor, wherein the capacitor may store a charge using a portion of
the electrical energy contained in the one or more input signals, and
wherein the capacitor may be configured to energize the power-on reset
circuit to reset the register contents when the implanted neural
stimulator loses power.

[0008] The at least one antenna may be configured to transmit, to the
separate antenna through electrical radiative coupling, one or more
stimulus feedback signals. The one or more circuits may be configured to
generate a stimulus feedback signal. The stimulus feedback signal may
indicate one or more parameters associated with the one or more
electrical pulses applied to the excitable tissue by the one or more
electrodes. The parameters may include the power being delivered to the
tissue and an impedance at the tissue.

[0009] The one or more circuits may include a current sensor configured to
sense an amount of current being delivered to the tissue and a voltage
sensor configured to sense a voltage being delivered to the tissue. The
current sensor may include a resistor placed in serial connection with an
anodic branch of the polarity routing switch network, and the anodic
portion of the electrical pulses may be transported over the anodic
branch. The current sensor and the voltage sensor are coupled to an
analog controlled carrier modulator, the modulator being configured to
communicate the sensed current and voltage to the separate antenna.

[0010] The at least one antenna may include a first antenna and a second
antenna. The first antenna may be configured to receive an input signal
containing the electrical energy. The second antenna may be configured to
transmit the stimulus feedback signal to the separate antenna through
electrical radiative coupling. The second antenna may be further
configured to receive an input signal containing the polarity assignment
information. The transmission frequency of the second antenna may be
higher than a resonant frequency of the first antenna. The transmission
frequency of the second antenna may be a second harmonic of the resonant
frequency of the first antenna. The transmission frequency and the
resonant frequency are in a range from about 300 MHz to about 6 GHz. The
at least one antenna may be between about 0.1 mm and about 7 cm in length
and between about 0.1 mm to about 3 mm in width. The at least one antenna
may be a dipole antenna.

[0011] The one or more circuits may additionally include a rectifying
circuit configured to rectify the input signal received by the first
antenna to generate the one or more electrical pulses. The rectifying
circuit may be coupled to a RC-timer to shape the one or more electrical
pulses. The rectifying circuit may include at least one full wave bridge
rectifier. The full wave bridge rectifier may include several diodes,
each of which may be less than 100 micrometers in length.

[0012] In another aspect, system includes a RF pulse generator module. The
RF pulse generator module includes an antenna module and one or more
circuits coupled to the antenna module.

[0013] The antenna module is configured to send one or more input signals
to at least one antenna in an implantable neural stimulator through
electrical radiative coupling. The one or more input signal contain
electrical energy and polarity assignment information that designates
polarity assignments of one or more electrodes in the implantable neural
stimulator. The implantable neural stimulator is configured to control an
electrode interface such that the electrodes have the polarities
designated by the polarity assignment information, create one or more
electrical pulses suitable for stimulation of neural tissue using the
electrical energy contained in the input signal, and supply the one or
more electrical pulses to the one or more electrodes through the
electrode interface such that the one or more electrodes apply the one or
more electrical pulses to neural tissue with the polarities designated by
the polarity assignment information. The antenna module is further
configured to receive one or more signals from the at least one antenna
in an implantable neural stimulator through the electrical radiative
coupling.

[0014] The one or more circuits are configured to generate the one or more
input signals and send the one or more input signals to the antenna
module; extract a stimulus feedback signal from one or more signals
received by the antenna module, the stimulus feedback signal being sent
by the implantable neural stimulator and indicating one or more
parameters of the one or more electrical pulses; and adjust parameters of
the input signal based on the stimulus feedback signal.

[0015] Implementations of this and other aspects may include the following
features. The antenna module may be configured to transmit portions of
the input signal containing electrical energy using a different carrier
frequency than portions of the input signal containing information
encoding the polarity assignments of one or more electrodes.

[0016] The antenna module may include a first antenna configured to
operate at a first frequency to transmit an input signal containing the
electrical energy and a second antenna configured to operate at a second
frequency to receive the one or more signals from the at least one
antenna of the implantable neural stimulator. The second frequency may
be, for example, a second harmonic frequency of the first frequency.

[0017] Various implementations may be inherently low in cost compared to
existing implantable neural modulation systems, and this may lead to
wider adoption of neural modulation therapy for patients in need as well
as reduction in overall cost to the healthcare system.

[0018] The details of one or more implementations are set forth in the
accompanying drawings and the description below. Other features, objects,
and advantages will be apparent from the description and drawings, and
from the claims.

DESCRIPTION OF DRAWINGS

[0019] FIG. 1 depicts a high-level diagram of an example of a wireless
neural stimulation system.

[0020] FIG. 2 depicts a detailed diagram of an example of the wireless
neural stimulation system.

[0021] FIG. 3 is a flowchart showing an example of the operation of the
wireless neural stimulator system.

[0022] FIG. 4 depicts a flow chart showing an example of the operation of
the system when the current level at the electrodes is above the
threshold limit.

[0023] FIG. 5 is a diagram showing examples of signals that may be used to
detect an impedance mismatch.

[0024] FIG. 6 is a diagram showing examples of signals that may be
employed during operation of the wireless neural stimulator system.

[0025] FIG. 7 is a flow chart showing a process for the user to control
the implantable wireless neural stimulator through an external programmer
in an open loop feedback system.

[0026] FIG. 8 is another example flow chart of a process for the user to
control the wireless stimulator with limitations on the lower and upper
limits of current amplitude.

[0027] FIG. 9 is yet another example flow chart of a process for the user
to control the wireless neural stimulator through preprogrammed parameter
settings.

[0028] FIG. 10 is still another example flow chart of a process for a low
battery state for the RF pulse generator module.

[0029] FIG. 11 is yet another example flow chart of a process for a
Manufacturer's Representative to program the implanted wireless neural
stimulator.

[0030] FIG. 12 is a circuit diagram showing an example of a wireless
neural stimulator.

[0031]FIG. 13 is a circuit diagram of another example of a wireless
neural stimulator.

[0032] FIG. 14 is a block diagram showing an example of control and
feedback functions of a wireless implantable neural stimulator.

[0033] FIG. 15 is a schematic showing an example of a wireless implantable
neural stimulator with components to implement control and feedback
functions.

[0034] FIG. 16 shows an example of a pulse waveform seen at the power
management circuitry of a wireless implantable neural stimulator.

[0035] FIG. 17 is a schematic of an example of a polarity routing switch
network.

[0036] FIGS. 18A and 18B, respectively show an example of a waveform
generated by a rectifying circuit of a wireless neural stimulator and the
corresponding spectrum.

[0037] FIG. 19 is a flow chart illustrating an example of operations of
control and feedback functions of a wireless implantable neural
stimulator.

DETAILED DESCRIPTION

[0038] In various implementations, a neural stimulation system may be used
to send electrical stimulation to targeted nerve tissue by using remote
radio frequency (RF) energy with neither cables nor inductive coupling to
power the passive implanted stimulator. The targeted nerve tissues may
be, for example, in the spinal column including the spinothalamic tracts,
dorsal horn, dorsal root ganglia, dorsal roots, dorsal column fibers, and
peripheral nerves bundles leaving the dorsal column or brainstem, as well
as any cranial nerves, abdominal, thoracic, or trigeminal ganglia nerves,
nerve bundles of the cerebral cortex, deep brain and any sensory or motor
nerves.

[0039] For instance, in some implementations, the neural stimulation
system may include a controller module, such as an RF pulse generator
module, and a passive implanted neural stimulator that contains one or
more dipole antennas, one or more circuits, and one or more electrodes in
contact with or in proximity to targeted neural tissue to facilitate
stimulation. The RF pulse generator module may include an antenna and may
be configured to transfer energy from the module antenna to the implanted
antennas. The one or more circuits of the implanted neural stimulator may
be configured to generate electrical pulses suitable for neural
stimulation using the transferred energy and to supply the electrical
pulses to the electrodes so that the pulses are applied to the neural
tissue. For instance, the one or more circuits may include wave
conditioning circuitry that rectifies the received RF signal (for
example, using a diode rectifier), transforms the RF energy to a low
frequency signal suitable for the stimulation of neural tissue, and
presents the resulting waveform to an electrode array. The one or more
circuits of the implanted neural stimulator may also include circuitry
for communicating information back to the RF pulse generator module to
facilitate a feedback control mechanism for stimulation parameter
control. For example, the implanted neural stimulator may send to the RF
pulse generator module a stimulus feedback signal that is indicative of
parameters of the electrical pulses, and the RF pulse generator module
may employ the stimulus feedback signal to adjust parameters of the
signal sent to the neural stimulator.

[0040] FIG. 1 depicts a high-level diagram of an example of a neural
stimulation system. The neural stimulation system may include four major
components, namely, a programmer module 102, a RF pulse generator module
106, a transmit (TX) antenna 110 (for example, a patch antenna, slot
antenna, or a dipole antenna), and an implanted wireless neural
stimulator 114. The programmer module 102 may be a computer device, such
as a smart phone, running a software application that supports a wireless
connection 114, such as Bluetooth®. The application can enable the
user to view the system status and diagnostics, change various
parameters, increase/decrease the desired stimulus amplitude of the
electrode pulses, and adjust feedback sensitivity of the RF pulse
generator module 106, among other functions.

[0041] The RF pulse generator module 106 may include communication
electronics that support the wireless connection 104, the stimulation
circuitry, and the battery to power the generator electronics. In some
implementations, the RF pulse generator module 106 includes the TX
antenna embedded into its packaging form factor while, in other
implementations, the TX antenna is connected to the RF pulse generator
module 106 through a wired connection 108 or a wireless connection (not
shown). The TX antenna 110 may be coupled directly to tissue to create an
electric field that powers the implanted neural stimulator module 114.
The TX antenna 110 communicates with the implanted neural stimulator
module 114 through an RF interface. For instance, the TX antenna 110
radiates an RF transmission signal that is modulated and encoded by the
RF pulse generator module 110. The implanted wireless neural stimulator
module 114 contains one or more antennas, such as dipole antenna(s), to
receive and transmit through RF interface 112. In particular, the
coupling mechanism between antenna 110 and the one or more antennas on
the implanted neural stimulation module 114 is electrical radiative
coupling and not inductive coupling. In other words, the coupling is
through an electric field rather than a magnetic field.

[0042] Through this electrical radiative coupling, the TX antenna 110 can
provide an input signal to the implanted neural stimulation module 114.
This input signal contains energy and may contain information encoding
stimulus waveforms to be applied at the electrodes of the implanted
neural stimulator module 114. In some implementations, the power level of
this input signal directly determines an applied amplitude (for example,
power, current, or voltage) of the one or more electrical pulses created
using the electrical energy contained in the input signal. Within the
implanted wireless neural stimulator 114 are components for demodulating
the RF transmission signal, and electrodes to deliver the stimulation to
surrounding neuronal tissue.

[0043] The RF pulse generator module 106 can be implanted subcutaneously,
or it can be worn external to the body. When external to the body, the RF
generator module 106 can be incorporated into a belt or harness design to
allow for electric radiative coupling through the skin and underlying
tissue to transfer power and/or control parameters to the implanted
neural stimulator module 114, which can be a passive stimulator. In
either event, receiver circuit(s) internal to the neural stimulator
module 114 can capture the energy radiated by the TX antenna 110 and
convert this energy to an electrical waveform. The receiver circuit(s)
may further modify the waveform to create an electrical pulse suitable
for the stimulation of neural tissue, and this pulse may be delivered to
the tissue via electrode pads.

[0044] In some implementations, the RF pulse generator module 106 can
remotely control the stimulus parameters (that is, the parameters of the
electrical pulses applied to the neural tissue) and monitor feedback from
the wireless neural stimulator module 114 based on RF signals received
from the implanted wireless neural stimulator module 114. A feedback
detection algorithm implemented by the RF pulse generator module 106 can
monitor data sent wirelessly from the implanted wireless neural
stimulator module 114, including information about the energy that the
implanted wireless neural stimulator module 114 is receiving from the RF
pulse generator and information about the stimulus waveform being
delivered to the electrode pads. In order to provide an effective therapy
for a given medical condition, the system can be tuned to provide the
optimal amount of excitation or inhibition to the nerve fibers by
electrical stimulation. A closed loop feedback control method can be used
in which the output signals from the implanted wireless neural stimulator
module 114 are monitored and used to determine the appropriate level of
neural stimulation current for maintaining effective neuronal activation,
or, in some cases, the patient can manually adjust the output signals in
an open loop control method.

[0045] FIG. 2 depicts a detailed diagram of an example of the neural
stimulation system. As depicted, the programming module 102 may comprise
user input system 202 and communication subsystem 208. The user input
system 221 may allow various parameter settings to be adjusted (in some
cases, in an open loop fashion) by the user in the form of instruction
sets. The communication subsystem 208 may transmit these instruction sets
(and other information) via the wireless connection 104, such as
Bluetooth or Wi-Fi, to the RF pulse generator module 106, as well as
receive data from module 106.

[0046] For instance, the programmer module 102, which can be utilized for
multiple users, such as a patient's control unit or clinician's
programmer unit, can be used to send stimulation parameters to the RF
pulse generator module 106. The stimulation parameters that can be
controlled may include pulse amplitude, pulse frequency, and pulse width
in the ranges shown in Table 1. In this context the term pulse refers to
the phase of the waveform that directly produces stimulation of the
tissue; the parameters of the charge-balancing phase (described below)
can similarly be controlled. The patient and/or the clinician can also
optionally control overall duration and pattern of treatment.

[0047] The implantable neural stimulator module 114 or RF pulse generator
module 114 may be initially programmed to meet the specific parameter
settings for each individual patient during the initial implantation
procedure. Because medical conditions or the body itself can change over
time, the ability to re-adjust the parameter settings may be beneficial
to ensure ongoing efficacy of the neural modulation therapy.

[0048] The programmer module 102 may be functionally a smart device and
associated application. The smart device hardware may include a CPU 206
and be used as a vehicle to handle touchscreen input on a graphical user
interface (GUI) 204, for processing and storing data.

[0049] The RF pulse generator module 106 may be connected via wired
connection 108 to an external TX antenna 110. Alternatively, both the
antenna and the RF pulse generator are located subcutaneously (not
shown).

[0050] The signals sent by RF pulse generator module 106 to the implanted
stimulator 114 may include both power and parameter-setting attributes in
regards to stimulus waveform, amplitude, pulse width, and frequency. The
RF pulse generator module 106 can also function as a wireless receiving
unit that receives feedback signals from the implanted stimulator module
114. To that end, the RF pulse generator module 106 may contain
microelectronics or other circuitry to handle the generation of the
signals transmitted to the stimulator module 114 as well as handle
feedback signals, such as those from the stimulator module 114. For
example, the RF pulse generator module 106 may comprise controller
subsystem 214, high-frequency oscillator 218, RF amplifier 216, a RF
switch, and a feedback subsystem 212.

[0052] The controller subsystem 214 may be used by the patient and/or the
clinician to control the stimulation parameter settings (for example, by
controlling the parameters of the signal sent from RF pulse generator
module 106 to neural stimulator module 114). These parameter settings can
affect, for example, the power, current level, or shape of the one or
more electrical pulses. The programming of the stimulation parameters can
be performed using the programming module 102, as described above, to set
the repetition rate, pulse width, amplitude, and waveform that will be
transmitted by RF energy to the receive (RX) antenna 238, typically a
dipole antenna (although other types may be used), in the wireless
implanted neural stimulator module 214. The clinician may have the option
of locking and/or hiding certain settings within the programmer
interface, thus limiting the patient's ability to view or adjust certain
parameters because adjustment of certain parameters may require detailed
medical knowledge of neurophysiology, neuroanatomy, protocols for neural
modulation, and safety limits of electrical stimulation.

[0053] The controller subsystem 214 may store received parameter settings
in the local memory subsystem 228, until the parameter settings are
modified by new input data received from the programming module 102. The
CPU 206 may use the parameters stored in the local memory to control the
pulse generator circuitry 236 to generate a stimulus waveform that is
modulated by a high frequency oscillator 218 in the range from 300 MHz to
8 GHz. The resulting RF signal may then be amplified by RF amplifier 226
and then sent through an RF switch 223 to the TX antenna 110 to reach
through depths of tissue to the RX antenna 238.

[0054] In some implementations, the RF signal sent by TX antenna 110 may
simply be a power transmission signal used by stimulator module 114 to
generate electric pulses. In other implementations, a telemetry signal
may also be transmitted to the stimulator module 114 to send instructions
about the various operations of the stimulator module 114. The telemetry
signal may be sent by the modulation of the carrier signal (through the
skin if external, or through other body tissues if the pulse generator
module 106 is implanted subcutaneously). The telemetry signal is used to
modulate the carrier signal (a high frequency signal) that is coupled
onto the implanted antenna(s) 238 and does not interfere with the input
received on the same lead to power the implant. In one embodiment the
telemetry signal and powering signal are combined into one signal, where
the RF telemetry signal is used to modulate the RF powering signal, and
thus the implanted stimulator is powered directly by the received
telemetry signal; separate subsystems in the stimulator harness the power
contained in the signal and interpret the data content of the signal.

[0055] The RF switch 223 may be a multipurpose device such as a dual
directional coupler, which passes the relatively high amplitude,
extremely short duration RF pulse to the TX antenna 110 with minimal
insertion loss while simultaneously providing two low-level outputs to
feedback subsystem 212; one output delivers a forward power signal to the
feedback subsystem 212, where the forward power signal is an attenuated
version of the RF pulse sent to the TX antenna 110, and the other output
delivers a reverse power signal to a different port of the feedback
subsystem 212, where reverse power is an attenuated version of the
reflected RF energy from the TX Antenna 110.

[0056] During the on-cycle time (when an RF signal is being transmitted to
stimulator 114), the RF switch 223 is set to send the forward power
signal to feedback subsystem. During the off-cycle time (when an RF
signal is not being transmitted to the stimulator module 114), the RF
switch 223 can change to a receiving mode in which the reflected RF
energy and/or RF signals from the stimulator module 114 are received to
be analyzed in the feedback subsystem 212.

[0057] The feedback subsystem 212 of the RF pulse generator module 106 may
include reception circuitry to receive and extract telemetry or other
feedback signals from the stimulator 114 and/or reflected RF energy from
the signal sent by TX antenna 110. The feedback subsystem may include an
amplifier 226, a filter 224, a demodulator 222, and an A/D converter 220.

[0058] The feedback subsystem 212 receives the forward power signal and
converts this high-frequency AC signal to a DC level that can be sampled
and sent to the controller subsystem 214. In this way the characteristics
of the generated RF pulse can be compared to a reference signal within
the controller subsystem 214. If a disparity (error) exists in any
parameter, the controller subsystem 214 can adjust the output to the RF
pulse generator 106. The nature of the adjustment can be, for example,
proportional to the computed error. The controller subsystem 214 can
incorporate additional inputs and limits on its adjustment scheme such as
the signal amplitude of the reverse power and any predetermined maximum
or minimum values for various pulse parameters.

[0059] The reverse power signal can be used to detect fault conditions in
the RF-power delivery system. In an ideal condition, when TX antenna 110
has perfectly matched impedance to the tissue that it contacts, the
electromagnetic waves generated from the RF pulse generator 106 pass
unimpeded from the TX antenna 110 into the body tissue. However, in
real-world applications a large degree of variability may exist in the
body types of users, types of clothing worn, and positioning of the
antenna 110 relative to the body surface. Since the impedance of the
antenna 110 depends on the relative permittivity of the underlying tissue
and any intervening materials, and also depends on the overall separation
distance of the antenna from the skin, in any given application there can
be an impedance mismatch at the interface of the TX antenna 110 with the
body surface. When such a mismatch occurs, the electromagnetic waves sent
from the RF pulse generator 106 are partially reflected at this
interface, and this reflected energy propagates backward through the
antenna feed.

[0060] The dual directional coupler RF switch 223 may prevent the
reflected RF energy propagating back into the amplifier 226, and may
attenuate this reflected RF signal and send the attenuated signal as the
reverse power signal to the feedback subsystem 212. The feedback
subsystem 212 can convert this high-frequency AC signal to a DC level
that can be sampled and sent to the controller subsystem 214. The
controller subsystem 214 can then calculate the ratio of the amplitude of
the reverse power signal to the amplitude of the forward power signal.
The ratio of the amplitude of reverse power signal to the amplitude level
of forward power may indicate severity of the impedance mismatch.

[0061] In order to sense impedance mismatch conditions, the controller
subsystem 214 can measure the reflected-power ratio in real time, and
according to preset thresholds for this measurement, the controller
subsystem 214 can modify the level of RF power generated by the RF pulse
generator 106. For example, for a moderate degree of reflected power the
course of action can be for the controller subsystem 214 to increase the
amplitude of RF power sent to the TX antenna 110, as would be needed to
compensate for slightly non-optimum but acceptable TX antenna coupling to
the body. For higher ratios of reflected power, the course of action can
be to prevent operation of the RF pulse generator 106 and set a fault
code to indicate that the TX antenna 110 has little or no coupling with
the body. This type of reflected-power fault condition can also be
generated by a poor or broken connection to the TX antenna. In either
case, it may be desirable to stop RF transmission when the
reflected-power ratio is above a defined threshold, because internally
reflected power can lead to unwanted heating of internal components, and
this fault condition means the system cannot deliver sufficient power to
the implanted wireless neural stimulator and thus cannot deliver therapy
to the user.

[0062] The controller 242 of the stimulator 114 may transmit informational
signals, such as a telemetry signal, through the antenna 238 to
communicate with the RF pulse generator module 106 during its receive
cycle. For example, the telemetry signal from the stimulator 114 may be
coupled to the modulated signal on the dipole antenna(s) 238, during the
on and off state of the transistor circuit to enable or disable a
waveform that produces the corresponding RF bursts necessary to transmit
to the external (or remotely implanted) pulse generator module 106. The
antenna(s) 238 may be connected to electrodes 254 in contact with tissue
to provide a return path for the transmitted signal. An A/D (not shown)
converter can be used to transfer stored data to a serialized pattern
that can be transmitted on the pulse modulated signal from the internal
antenna(s) 238 of the neural stimulator.

[0063] A telemetry signal from the implanted wireless neural stimulator
module 114 may include stimulus parameters such as the power or the
amplitude of the current that is delivered to the tissue from the
electrodes. The feedback signal can be transmitted to the RF pulse
generator module 116 to indicate the strength of the stimulus at the
nerve bundle by means of coupling the signal to the implanted RX antenna
238, which radiates the telemetry signal to the external (or remotely
implanted) RF pulse generator module 106. The feedback signal can include
either or both an analog and digital telemetry pulse modulated carrier
signal. Data such as stimulation pulse parameters and measured
characteristics of stimulator performance can be stored in an internal
memory device within the implanted neural stimulator 114, and sent on the
telemetry signal. The frequency of the carrier signal may be in the range
of at 300 MHz to 8 GHz.

[0064] In the feedback subsystem 212, the telemetry signal can be down
modulated using demodulator 222 and digitized by being processed through
an analog to digital (A/D) converter 220. The digital telemetry signal
may then be routed to a CPU 230 with embedded code, with the option to
reprogram, to translate the signal into a corresponding current
measurement in the tissue based on the amplitude of the received signal.
The CPU 230 of the controller subsystem 214 can compare the reported
stimulus parameters to those held in local memory 228 to verify the
stimulator(s) 114 delivered the specified stimuli to tissue. For example,
if the stimulator reports a lower current than was specified, the power
level from the RF pulse generator module 106 can be increased so that the
implanted neural stimulator 114 will have more available power for
stimulation. The implanted neural stimulator 114 can generate telemetry
data in real time, for example, at a rate of 8 kbits per second. All
feedback data received from the implanted lead module 114 can be logged
against time and sampled to be stored for retrieval to a remote
monitoring system accessible by the health care professional for trending
and statistical correlations.

[0065] The sequence of remotely programmable RF signals received by the
internal antenna(s) 238 may be conditioned into waveforms that are
controlled within the implantable stimulator 114 by the control subsystem
242 and routed to the appropriate electrodes 254 that are placed in
proximity to the tissue to be stimulated. For instance, the RF signal
transmitted from the RF pulse generator module 106 may be received by RX
antenna 238 and processed by circuitry, such as waveform conditioning
circuitry 240, within the implanted wireless neural stimulator module 114
to be converted into electrical pulses applied to the electrodes 254
through electrode interface 252. In some implementations, the implanted
stimulator 114 contains between two to sixteen electrodes 254.

[0066] The waveform conditioning circuitry 240 may include a rectifier
244, which rectifies the signal received by the RX antenna 238. The
rectified signal may be fed to the controller 242 for receiving encoded
instructions from the RF pulse generator module 106. The rectifier signal
may also be fed to a charge balance component 246 that is configured to
create one or more electrical pulses based such that the one or more
electrical pulses result in a substantially zero net charge at the one or
more electrodes (that is, the pulses are charge balanced). The
charge-balanced pulses are passed through the current limiter 248 to the
electrode interface 252, which applies the pulses to the electrodes 254
as appropriate.

[0067] The current limiter 248 insures the current level of the pulses
applied to the electrodes 254 is not above a threshold current level. In
some implementations, an amplitude (for example, current level, voltage
level, or power level) of the received RF pulse directly determines the
amplitude of the stimulus. In this case, it may be particularly
beneficial to include current limiter 248 to prevent excessive current or
charge being delivered through the electrodes, although current limiter
248 may be used in other implementations where this is not the case.
Generally, for a given electrode having several square millimeters
surface area, it is the charge per phase that should be limited for
safety (where the charge delivered by a stimulus phase is the integral of
the current). But, in some cases, the limit can instead be placed on the
current, where the maximum current multiplied by the maximum possible
pulse duration is less than or equal to the maximum safe charge. More
generally, the limiter 248 acts as a charge limiter that limits a
characteristic (for example, current or duration) of the electrical
pulses so that the charge per phase remains below a threshold level
(typically, a safe-charge limit).

[0068] In the event the implanted wireless neural stimulator 114 receives
a "strong" pulse of RF power sufficient to generate a stimulus that would
exceed the predetermined safe-charge limit, the current limiter 248 can
automatically limit or "clip" the stimulus phase to maintain the total
charge of the phase within the safety limit. The current limiter 248 may
be a passive current limiting component that cuts the signal to the
electrodes 254 once the safe current limit (the threshold current level)
is reached. Alternatively, or additionally, the current limiter 248 may
communicate with the electrode interface 252 to turn off all electrodes
254 to prevent tissue damaging current levels.

[0069] A clipping event may trigger a current limiter feedback control
mode. The action of clipping may cause the controller to send a threshold
power data signal to the pulse generator 106. The feedback subsystem 212
detects the threshold power signal and demodulates the signal into data
that is communicated to the controller subsystem 214. The controller
subsystem 214 algorithms may act on this current-limiting condition by
specifically reducing the RF power generated by the RF pulse generator,
or cutting the power completely. In this way, the pulse generator 106 can
reduce the RF power delivered to the body if the implanted wireless
neural stimulator 114 reports it is receiving excess RF power.

[0070] The controller 250 of the stimulator 205 may communicate with the
electrode interface 252 to control various aspects of the electrode setup
and pulses applied to the electrodes 254. The electrode interface 252 may
act as a multiplex and control the polarity and switching of each of the
electrodes 254. For instance, in some implementations, the wireless
stimulator 106 has multiple electrodes 254 in contact with tissue, and
for a given stimulus the RF pulse generator module 106 can arbitrarily
assign one or more electrodes to 1) act as a stimulating electrode, 2)
act as a return electrode, or 3) be inactive by communication of
assignment sent wirelessly with the parameter instructions, which the
controller 250 uses to set electrode interface 252 as appropriate. It may
be physiologically advantageous to assign, for example, one or two
electrodes as stimulating electrodes and to assign all remaining
electrodes as return electrodes.

[0071] Also, in some implementations, for a given stimulus pulse, the
controller 250 may control the electrode interface 252 to divide the
current arbitrarily (or according to instructions from pulse generator
module 106) among the designated stimulating electrodes. This control
over electrode assignment and current control can be advantageous because
in practice the electrodes 254 may be spatially distributed along various
neural structures, and through strategic selection of the stimulating
electrode location and the proportion of current specified for each
location, the aggregate current distribution in tissue can be modified to
selectively activate specific neural targets. This strategy of current
steering can improve the therapeutic effect for the patient.

[0072] In another implementation, the time course of stimuli may be
arbitrarily manipulated. A given stimulus waveform may be initiated at a
time T_start and terminated at a time T_final, and this time course may
be synchronized across all stimulating and return electrodes; further,
the frequency of repetition of this stimulus cycle may be synchronous for
all the electrodes. However, controller 250, on its own or in response to
instructions from pulse generator 106, can control electrode interface
252 to designate one or more subsets of electrodes to deliver stimulus
waveforms with non-synchronous start and stop times, and the frequency of
repetition of each stimulus cycle can be arbitrarily and independently
specified.

[0073] For example, a stimulator having eight electrodes may be configured
to have a subset of five electrodes, called set A, and a subset of three
electrodes, called set B. Set A might be configured to use two of its
electrodes as stimulating electrodes, with the remainder being return
electrodes. Set B might be configured to have just one stimulating
electrode. The controller 250 could then specify that set A deliver a
stimulus phase with 3 mA current for a duration of 200 us followed by a
400 us charge-balancing phase. This stimulus cycle could be specified to
repeat at a rate of 60 cycles per second. Then, for set B, the controller
250 could specify a stimulus phase with 1 mA current for duration of 500
us followed by a 800 us charge-balancing phase. The repetition rate for
the set-B stimulus cycle can be set independently of set A, say for
example it could be specified at 25 cycles per second. Or, if the
controller 250 was configured to match the repetition rate for set B to
that of set A, for such a case the controller 250 can specify the
relative start times of the stimulus cycles to be coincident in time or
to be arbitrarily offset from one another by some delay interval.

[0074] In some implementations, the controller 250 can arbitrarily shape
the stimulus waveform amplitude, and may do so in response to
instructions from pulse generator 106. The stimulus phase may be
delivered by a constant-current source or a constant-voltage source, and
this type of control may generate characteristic waveforms that are
static, e.g. a constant-current source generates a characteristic
rectangular pulse in which the current waveform has a very steep rise, a
constant amplitude for the duration of the stimulus, and then a very
steep return to baseline. Alternatively, or additionally, the controller
250 can increase or decrease the level of current at any time during the
stimulus phase and/or during the charge-balancing phase. Thus, in some
implementations, the controller 250 can deliver arbitrarily shaped
stimulus waveforms such as a triangular pulse, sinusoidal pulse, or
Gaussian pulse for example. Similarly, the charge-balancing phase can be
arbitrarily amplitude-shaped, and similarly a leading anodic pulse (prior
to the stimulus phase) may also be amplitude-shaped.

[0075] As described above, the stimulator 114 may include a
charge-balancing component 246. Generally, for constant current
stimulation pulses, pulses should be charge balanced by having the amount
of cathodic current should equal the amount of anodic current, which is
typically called biphasic stimulation. Charge density is the amount of
current times the duration it is applied, and is typically expressed in
the units uC/cm2. In order to avoid the irreversible electrochemical
reactions such as pH change, electrode dissolution as well as tissue
destruction, no net charge should appear at the electrode-electrolyte
interface, and it is generally acceptable to have a charge density less
than 30 uC/cm2. Biphasic stimulating current pulses ensure that no
net charge appears at the electrode after each stimulation cycle and the
electrochemical processes are balanced to prevent net dc currents. Neural
stimulator 114 may be designed to ensure that the resulting stimulus
waveform has a net zero charge. Charge balanced stimuli are thought to
have minimal damaging effects on tissue by reducing or eliminating
electrochemical reaction products created at the electrode-tissue
interface.

[0076] A stimulus pulse may have a negative-voltage or current, called the
cathodic phase of the waveform. Stimulating electrodes may have both
cathodic and anodic phases at different times during the stimulus cycle.
An electrode that delivers a negative current with sufficient amplitude
to stimulate adjacent neural tissue is called a "stimulating electrode."
During the stimulus phase the stimulating electrode acts as a current
sink. One or more additional electrodes act as a current source and these
electrodes are called "return electrodes." Return electrodes are placed
elsewhere in the tissue at some distance from the stimulating electrodes.
When a typical negative stimulus phase is delivered to tissue at the
stimulating electrode, the return electrode has a positive stimulus
phase. During the subsequent charge-balancing phase, the polarities of
each electrode are reversed.

[0077] In some implementations, the charge balance component 246 uses a
blocking capacitor(s) placed electrically in series with the stimulating
electrodes and body tissue, between the point of stimulus generation
within the stimulator circuitry and the point of stimulus delivery to
tissue. In this manner, a resistor-capacitor (RC) network may be formed.
In a multi-electrode stimulator, one charge-balance capacitor(s) may be
used for each electrode or a centralized capacitor(s) may be used within
the stimulator circuitry prior to the point of electrode selection. The
RC network can block direct current (DC), however it can also prevent
low-frequency alternating current (AC) from passing to the tissue. The
frequency below which the series RC network essentially blocks signals is
commonly referred to as the cutoff frequency, and in one embodiment the
design of the stimulator system may ensure the cutoff frequency is not
above the fundamental frequency of the stimulus waveform. In this
embodiment of the present invention, the wireless stimulator may have a
charge-balance capacitor with a value chosen according to the measured
series resistance of the electrodes and the tissue environment in which
the stimulator is implanted. By selecting a specific capacitance value
the cutoff frequency of the RC network in this embodiment is at or below
the fundamental frequency of the stimulus pulse.

[0078] In other implementations, the cutoff frequency may be chosen to be
at or above the fundamental frequency of the stimulus, and in this
scenario the stimulus waveform created prior to the charge-balance
capacitor, called the drive waveform, may be designed to be
non-stationary, where the envelope of the drive waveform is varied during
the duration of the drive pulse. For example, in one embodiment, the
initial amplitude of the drive waveform is set at an initial amplitude
Vi, and the amplitude is increased during the duration of the pulse until
it reaches a final value k*Vi. By changing the amplitude of the drive
waveform over time, the shape of the stimulus waveform passed through the
charge-balance capacitor is also modified. The shape of the stimulus
waveform may be modified in this fashion to create a physiologically
advantageous stimulus.

[0079] In some implementations, the wireless neural stimulator module 114
may create a drive-waveform envelope that follows the envelope of the RF
pulse received by the receiving dipole antenna(s) 238. In this case, the
RF pulse generator module 106 can directly control the envelope of the
drive waveform within the wireless neural stimulator 114, and thus no
energy storage may be required inside the stimulator itself. In this
implementation, the stimulator circuitry may modify the envelope of the
drive waveform or may pass it directly to the charge-balance capacitor
and/or electrode-selection stage.

[0080] In some implementations, the implanted neural stimulator 114 may
deliver a single-phase drive waveform to the charge balance capacitor or
it may deliver multiphase drive waveforms. In the case of a single-phase
drive waveform, for example, a negative-going rectangular pulse, this
pulse comprises the physiological stimulus phase, and the charge-balance
capacitor is polarized (charged) during this phase. After the drive pulse
is completed, the charge balancing function is performed solely by the
passive discharge of the charge-balance capacitor, where is dissipates
its charge through the tissue in an opposite polarity relative to the
preceding stimulus. In one implementation, a resistor within the
stimulator facilitates the discharge of the charge-balance capacitor. In
some implementations, using a passive discharge phase, the capacitor may
allow virtually complete discharge prior to the onset of the subsequent
stimulus pulse.

[0081] In the case of multiphase drive waveforms the wireless stimulator
may perform internal switching to pass negative-going or positive-going
pulses (phases) to the charge-balance capacitor. These pulses may be
delivered in any sequence and with varying amplitudes and waveform shapes
to achieve a desired physiological effect. For example, the stimulus
phase may be followed by an actively driven charge-balancing phase,
and/or the stimulus phase may be preceded by an opposite phase. Preceding
the stimulus with an opposite-polarity phase, for example, can have the
advantage of reducing the amplitude of the stimulus phase required to
excite tissue.

[0082] In some implementations, the amplitude and timing of stimulus and
charge-balancing phases is controlled by the amplitude and timing of RF
pulses from the RF pulse generator module 106, and in others this control
may be administered internally by circuitry onboard the wireless
stimulator 114, such as controller 250. In the case of onboard control,
the amplitude and timing may be specified or modified by data commands
delivered from the pulse generator module 106.

[0083] FIG. 3 is a flowchart showing an example of an operation of the
neural stimulator system. In block 302, the wireless neural stimulator
114 is implanted in proximity to nerve bundles and is coupled to the
electric field produced by the TX antenna 110. That is, the pulse
generator module 106 and the TX antenna 110 are positioned in such a way
(for example, in proximity to the patient) that the TX antenna 110 is
electrically radiatively coupled with the implanted RX antenna 238 of the
neural stimulator 114. In certain implementations, both the antenna 110
and the RF pulse generator 106 are located subcutaneously. In other
implementations, the antenna 110 and the RF pulse generator 106 are
located external to the patient's body. In this case, the TX antenna 110
may be coupled directly to the patient's skin.

[0084] Energy from the RF pulse generator is radiated to the implanted
wireless neural stimulator 114 from the antenna 110 through tissue, as
shown in block 304. The energy radiated may be controlled by the
Patient/Clinician Parameter inputs in block 301. In some instances, the
parameter settings can be adjusted in an open loop fashion by the patient
or clinician, who would adjust the parameter inputs in block 301 to the
system.

[0085] The wireless implanted stimulator 114 uses the received energy to
generate electrical pulses to be applied to the neural tissue through the
electrodes 238. For instance, the stimulator 114 may contain circuitry
that rectifies the received RF energy and conditions the waveform to
charge balance the energy delivered to the electrodes to stimulate the
targeted nerves or tissues, as shown in block 306. The implanted
stimulator 114 communicates with the pulse generator 106 by using antenna
238 to send a telemetry signal, as shown in block 308. The telemetry
signal may contain information about parameters of the electrical pulses
applied to the electrodes, such as the impedance of the electrodes,
whether the safe current limit has been reached, or the amplitude of the
current that is presented to the tissue from the electrodes.

[0086] In block 310, the RF pulse generator 106 detects amplifies, filters
and modulates the received telemetry signal using amplifier 226, filter
224, and demodulator 222, respectively. The A/D converter 230 then
digitizes the resulting analog signal, as shown in 312. The digital
telemetry signal is routed to CPU 230, which determines whether the
parameters of the signal sent to the stimulator 114 need to be adjusted
based on the digital telemetry signal. For instance, in block 314, the
CPU 230 compares the information of the digital signal to a look-up
table, which may indicate an appropriate change in stimulation
parameters. The indicated change may be, for example, a change in the
current level of the pulses applied to the electrodes. As a result, the
CPU may change the output power of the signal sent to stimulator 114 so
as to adjust the current applied by the electrodes 254, as shown in block
316.

[0087] Thus, for instance, the CPU 230 may adjust parameters of the signal
sent to the stimulator 114 every cycle to match the desired current
amplitude setting programmed by the patient, as shown in block 318. The
status of the stimulator system may be sampled in real time at a rate of
8 kbits per second of telemetry data. All feedback data received from the
stimulator 114 can be maintained against time and sampled per minute to
be stored for download or upload to a remote monitoring system accessible
by the health care professional for trending and statistical correlations
in block 318. If operated in an open loop fashion, the stimulator system
operation may be reduced to just the functional elements shown in blocks
302, 304, 306, and 308, and the patient uses their judgment to adjust
parameter settings rather than the closed looped feedback from the
implanted device.

[0088] FIG. 4 depicts a flow chart showing an example of an operation of
the system when the current level at the electrodes 254 is above a
threshold limit. In certain instances, the implanted wireless neural
stimulator 114 may receive an input power signal with a current level
above an established safe current limit, as shown in block 402. For
instance, the current limiter 248 may determine the current is above an
established tissue-safe limit of amperes, as shown in block 404. If the
current limiter senses that the current is above the threshold, it may
stop the high-power signal from damaging surrounding tissue in contact
with the electrodes as shown in block 406, the operations of which are as
described above in association with FIG. 2.

[0089] A capacitor may store excess power, as shown in block 408. When the
current limiter senses the current is above the threshold, the controller
250 may use the excess power available to transmit a small 2-bit data
burst back to the RF pulse generator 106, as shown in block 410. The
2-bit data burst may be transmitted through the implanted wireless neural
stimulator's antenna(s) 238 during the RF pulse generator's receive
cycle, as shown in block 412. The RF pulse generator antenna 110 may
receive the 2-bit data burst during its receive cycle, as shown in block
414, at a rate of 8 kbps, and may relay the data burst back to the RF
pulse generator's feedback subsystem 212 which is monitoring all reverse
power, as shown in block 416. The CPU 230 may analyze signals from
feedback subsystem 202, as shown in block 418 and if there is no data
burst present, no changes may be made to the stimulation parameters, as
shown in block 420. If the data burst is present in the analysis, the CPU
230 can cut all transmission power for one cycle, as shown in block 422.

[0090] If the data burst continues, the RF pulse generator 106 may push a
"proximity power danger" notification to the application on the
programmer module 102, as shown in block 424. This proximity danger
notification occurs because the RF pulse generator has ceased its
transmission of power. This notification means an unauthorized form of
energy is powering the implant above safe levels. The application may
alert the user of the danger and that the user should leave the immediate
area to resume neural modulation therapy, as shown in block 426. If after
one cycle the data burst has stopped, the RF pulse generator 106 may
slowly ramp up the transmission power in increments, for example from 5%
to 75% of previous current amplitude levels, as shown in block 428. The
user can then manually adjust current amplitude level to go higher at the
user's own risk. During the ramp up, the RF pulse generator 106 may
notify the application of its progress and the application may notify the
user that there was an unsafe power level and the system is ramping back
up, as shown in block 430.

[0091] FIG. 5 is a diagram showing examples of signals that may be used to
detect an impedance mismatch. As described above, a forward power signal
and a reverse power signal may be used to detect an impedance mismatch.
For instance, a RF pulse 502 generated by the RF pulse generator may pass
through a device such as a dual directional coupler to the TX antenna
110. The TX antenna 110 then radiates the RF signal into the body, where
the energy is received by the implanted wireless neural stimulator 114
and converted into a tissue-stimulating pulse. The coupler passes an
attenuated version of this RF signal, forward power 510, to feedback
subsystem 212. The feedback subsystem 212 demodulates the AC signal and
computes the amplitude of the forward RF power, and this data is passed
to controller subsystem 214. Similarly the dual directional coupler (or
similar component) also receives RF energy reflected back from the TX
antenna 110 and passes an attenuated version of this RF signal, reverse
power 512, to feedback subsystem 212. The feedback subsystem 212
demodulates the AC signal and computes the amplitude of the reflected RF
power, and this data is passed to controller subsystem 214.

[0092] In the optimal case, when the TX antenna 110 may be perfectly
impedance-matched to the body so that the RF energy passes unimpeded
across the interface of the TX antenna 110 to the body, and no RF energy
is reflected at the interface. Thus, in this optimal case, the reverse
power 512 may have close to zero amplitude as shown by signal 504, and
the ratio of reverse power 512 to forward power 510 is zero. In this
circumstance, no error condition exists, and the controller 214 sets a
system message that operation is optimal.

[0093] In practice, the impedance match of the TX antenna 204 to the body
may not be optimal, and some energy of the RF pulse 502 is reflected from
the interface of the TX antenna 110 and the body. This can occur for
example if the TX antenna 110 is held somewhat away from the skin by a
piece of clothing. This non-optimal antenna coupling causes a small
portion of the forward RF energy to be reflected at the interface, and
this is depicted as signal 506. In this case, the ratio of reverse power
512 to forward power 510 is small, but a small ratio implies that most of
the RF energy is still radiated from the TX antenna 110, so this
condition is acceptable within the control algorithm. This determination
of acceptable reflection ratio may be made within controller subsystem
214 based upon a programmed threshold, and the controller subsystem 214
may generate a low-priority alert to be sent to the user interface. In
addition, the controller subsystem 214 sensing the condition of a small
reflection ratio, may moderately increase the amplitude of the RF pulse
502 to compensate for the moderate loss of forward energy transfer to the
implanted wireless neural stimulator 114.

[0094] During daily operational use, the TX antenna 110 might be
accidentally removed from the body entirely, in which case the TX antenna
will have very poor coupling to the body (if any). In this or other
circumstances, a relatively high proportion of the RF pulse energy is
reflected as signal 508 from the TX antenna 110 and fed backward into the
RF-powering system. Similarly, this phenomenon can occur if the
connection to the TX antenna is physically broken, in which case
virtually 100% of the RF energy is reflected backward from the point of
the break. In such cases, the ratio of reverse power 512 to forward power
510 is very high, and the controller subsystem 214 will determine the
ratio has exceeded the threshold of acceptance. In this case, the
controller subsystem 214 may prevent any further RF pulses from being
generated. The shutdown of the RF pulse generator module 106 may be
reported to the user interface to inform the user that stimulation
therapy cannot be delivered.

[0095] FIG. 6 is a diagram showing examples of signals that may be
employed during operation of the neural stimulator system. According to
some implementations, the amplitude of the RF pulse 602 received by the
implanted wireless neural stimulator 114 can directly control the
amplitude of the stimulus 630 delivered to tissue. The duration of the RF
pulse 608 corresponds to the specified pulse width of the stimulus 630.
During normal operation the RF pulse generator module 106 sends an RF
pulse waveform 602 via TX antenna 110 into the body, and RF pulse
waveform 608 may represent the corresponding RF pulse received by
implanted wireless neural stimulator 114. In this instance the received
power has an amplitude suitable for generating a safe stimulus pulse 630.
The stimulus pulse 630 is below the safety threshold 626, and no error
condition exists. In another example, the attenuation between the TX
antenna 110 and the implanted wireless neural stimulator 114 has been
unexpectedly reduced, for example due to the user repositioning the TX
antenna 110. This reduced attenuation can lead to increased amplitude in
the RF pulse waveform 612 being received at the neural stimulator 114.
Although the RF pulse 602 is generated with the same amplitude as before,
the improved RF coupling between the TX antenna 110 and the implanted
wireless neural stimulator 114 can cause the received RF pulse 612 to be
larger in amplitude. Implanted wireless neural stimulator 114 in this
situation may generate a larger stimulus 632 in response to the increase
in received RF pulse 612. However, in this example, the received power
612 is capable of generating a stimulus 632 that exceeds the prudent
safety limit for tissue. In this situation, the current limiter feedback
control mode can operate to clip the waveform of the stimulus pulse 632
such that the stimulus delivered is held within the predetermined safety
limit 626. The clipping event 628 may be communicated through the
feedback subsystem 212 as described above, and subsequently controller
subsystem 214 can reduce the amplitude specified for the RF pulse. As a
result, the subsequent RF pulse 604 is reduced in amplitude, and
correspondingly the amplitude of the received RF pulse 616 is reduced to
a suitable level (non-clipping level). In this fashion, the current
limiter feedback control mode may operate to reduce the RF power
delivered to the body if the implanted wireless neural stimulator 114
receives excess RF power.

[0096] In another example, the RF pulse waveform 606 depicts a higher
amplitude RF pulse generated as a result of user input to the user
interface. In this circumstance, the RF pulse 620 received by the
implanted wireless neural stimulator 14 is increased in amplitude, and
similarly current limiter feedback mode operates to prevent stimulus 636
from exceeding safety limit 626. Once again, this clipping event 628 may
be communicated through the feedback subsystem 212, and subsequently
controller subsystem 214 may reduce the amplitude of the RF pulse, thus
overriding the user input. The reduced RF pulse 604 can produce
correspondingly smaller amplitudes of the received waveforms 616, and
clipping of the stimulus current may no longer be required to keep the
current within the safety limit. In this fashion, the current limiter
feedback may reduce the RF power delivered to the body if the implanted
wireless neural stimulator 114 reports it is receiving excess RF power.

[0097] FIG. 7 is a flow chart showing a process for the user to control
the implantable wireless neural stimulator through the programmer in an
open loop feedback system. In one implementation of the system, the user
has a wireless neural stimulator implanted in their body, the RF pulse
generator 106 sends the stimulating pulse power wirelessly to the
stimulator 114, and an application on the programmer module 102 (for
example, a smart device) is communicating with the RF pulse generator
106. In this implementation, if a user wants to observe the current
status of the functioning pulse generator, as shown in block 702, the
user may open the application, as shown in block 704. The application can
use Bluetooth protocols built into the smart device to interrogate the
pulse generator, as shown in block 706. The RF pulse generator 106 may
authenticate the identity of the smart device and serialized patient
assigned secure iteration of the application, as shown in block 708. The
authentication process may utilize a unique key to the patient specific
RF pulse generator serial number. The application can be customized with
the patient specific unique key through the Manufacturer Representative
who has programmed the initial patient settings for the stimulation
system, as shown in block 720. If the RF pulse generator rejects the
authentication it may inform the application that the code is invalid, as
shown in block 718 and needs the authentication provided by the
authorized individual with security clearance from the device
manufacturer, known as the "Manufacturer's Representative," as shown in
block 722. In an implementation, only the Manufacturer's Representative
can have access to the security code needed to change the application's
stored RF pulse generator unique ID. If the RF pulse generator
authentication system passes, the pulse generator module 106 sends back
all of the data that has been logged since the last sync, as shown in
block 710. The application may then register the most current information
and transmit the information to a 3rd party in a secure fashion, as shown
in 712. The application may maintain a database that logs all system
diagnostic results and values, the changes in settings by the user and
the feedback system, and the global runtime history, as shown in block
714. The application may then display relevant data to the user, as shown
in block 716; including the battery capacity, current program parameter,
running time, pulse width, frequency, amplitude, and the status of the
feedback system.

[0098] FIG. 8 is another example flow chart of a process for the user to
control the wireless stimulator with limitations on the lower and upper
limits of current amplitude. The user wants to change the amplitude of
the stimulation signal, as shown in block 802. The user may open the
application, as show in block 704 and the application may go through the
process described in FIG. 7 to communicate with the RF pulse generator,
authenticate successfully, and display the current status to the user, as
shown in block 804. The application displays the stimulation amplitude as
the most prevalent changeable interface option and displays two arrows
with which the user can adjust the current amplitude. The user may make a
decision based on their need for more or less stimulation in accordance
with their pain levels, as shown in block 806. If the user chooses to
increase the current amplitude, the user may press the up arrow on the
application screen, as shown in block 808. The application can include
safety maximum limiting algorithms, so if a request to increase current
amplitude is recognized by the application as exceeding the preset safety
maximum, as shown in block 810, then the application will display an
error message, as shown in block 812 and will not communicate with the RF
pulse generator module 106. If the user presses the up arrow, as shown in
block 808 and the current amplitude request does not exceed the current
amplitude maximum allowable value, then the application will send
instructions to the RF pulse generator module 106 to increase amplitude,
as shown in block 814. The RF pulse generator module 106 may then attempt
to increase the current amplitude of stimulation, as shown in block 816.
If the RF pulse generator is successful at increasing the current
amplitude, the RF pulse generator module 106 may perform a short
vibration to physically confirm with the user that the amplitude is
increased, as shown in block 818. The RF pulse generator module 106 can
also send back confirmation of increased amplitude to the application, as
shown in block 820, and then the application may display the updated
current amplitude level, as shown in block 822.

[0099] If the user decides to decrease the current amplitude level in
block 806, the user can press the down arrow on the application, as shown
in block 828. If the current amplitude level is already at zero, the
application recognizes that the current amplitude cannot be decreased any
further, as shown in block 830 and displays an error message to the user
without communicating any data to the RF pulse generator, as shown in
block 832. If the current amplitude level is not at zero, the application
can send instructions to the RF pulse generator module 106 to decrease
current amplitude level accordingly, as shown in block 834. The RF pulse
generator may then attempt to decrease current amplitude level of
stimulation RF pulse generator module 106 and, if successful, the RF
pulse generator module 106 may perform a short vibration to physically
confirm to the user that the current amplitude level has been decreased,
as shown in block 842. The RF pulse generator module 106 can send back
confirmation of the decreased current amplitude level to the application,
as shown in block 838. The application then may display the updated
current amplitude level, as indicated by block 840. If the current
amplitude level decrease or increase fails, the RF pulse generator module
106 can perform a series of short vibrations to alert user, and send an
error message to the application, as shown in block 824. The application
receives the error and may display the data for the user's benefit, as
shown in block 826.

[0100] FIG. 9 is yet another example flow chart of a process for the user
to control the wireless neural stimulator 114 through preprogrammed
parameter settings. The user wants to change the parameter program, as
indicated by block 902. When the user is implanted with a wireless neural
stimulator or when the user visits the doctor, the Manufacturer's
Representative may determine and provide the patient/user RF pulse
generator with preset programs that have different stimulation parameters
that will be used to treat the user. The user will then able to switch
between the various parameter programs as needed. The user can open the
application on their smart device, as indicated by block 704, which first
follows the process described in FIG. 7, communicating with the RF pulse
generator module 106, authenticating successfully, and displaying the
current status of the RF pulse generator module 106, including the
current program parameter settings, as indicated by block 812. In this
implementation, through the user interface of the application, the user
can select the program that they wish to use, as shown by block 904. The
application may then access a library of pre-programmed parameters that
have been approved by the Manufacturer's Representative for the user to
interchange between as desired and in accordance with the management of
their indication, as indicated by block 906. A table can be displayed to
the user, as shown in block 908 and each row displays a program's
codename and lists its basic parameter settings, as shown in block 910,
which includes but is not limited to: pulse width, frequency, cycle
timing, pulse shape, duration, feedback sensitivity, as shown in block
912. The user may then select the row containing the desired parameter
preset program to be used, as shown in block 912. The application can
send instructions to the RF pulse generator module 106 to change the
parameter settings, as shown in block 916. The RF pulse generator module
106 may attempt to change the parameter settings 154. If the parameter
settings are successfully changed, the RF pulse generator module 106 can
perform a unique vibration pattern to physically confirm with the user
that the parameter settings were changed, as shown in block 920. Also,
the RF pulse generator module 106 can send back confirmation to the
application that the parameter change has been successful, as shown in
block 922, and the application may display the updated current program,
as shown in block 924. If the parameter program change has failed, the RF
pulse generator module 106 may perform a series of short vibrations to
alert the user, and send an error message to the application, as shown in
block 926, which receives the error and may display to the user, as shown
in block 928.

[0101] FIG. 10 is still another example flow chart of a process for a low
battery state for the RF pulse generator module 106. In this
implementation, the RF pulse generator module's remaining battery power
level is recognized as low, as shown in block 1002. The RF pulse
generator module 106 regularly interrogates the power supply battery
subsystem 210 about the current power and the RF pulse generator
microprocessor asks the battery if its remaining power is below
threshold, as shown in block 1004. If the battery's remaining power is
above the threshold, the RF pulse generator module 106 may store the
current battery status to be sent to the application during the next
sync, as shown in block 1006. If the battery's remaining power is below
threshold the RF pulse generator module 106 may push a low-battery
notification to the application, as shown in block 1008. The RF pulse
generator module 106 may always perform one sequence of short vibrations
to alert the user of an issue and send the application a notification, as
shown in block 1010. If there continues to be no confirmation of the
application receiving the notification then the RF pulse generator can
continue to perform short vibration pulses to notify user, as shown in
block 1010. If the application successfully receives the notification, it
may display the notification and may need user acknowledgement, as shown
in block 1012. If, for example, one minute passes without the
notification message on the application being dismissed the application
informs the RF pulse generator module 106 about lack of human
acknowledgement, as shown in block 1014, and the RF pulse generator
module 106 may begin to perform the vibration pulses to notify the user,
as shown in block 1010. If the user dismisses the notification, the
application may display a passive notification to switch the battery, as
shown in block 1016. If a predetermined amount of time passes, such as
five minutes for example, without the battery being switched, the
application can inform the RF pulse generator module 106 of the lack of
human acknowledgement, as shown in block 1014 and the RF pulse generator
module 106 may perform vibrations, as shown in block 1010. If the RF
pulse generator module battery is switched, the RF pulse generator module
106 reboots and interrogates the battery to assess power remaining, as
shown in block 1018. If the battery's power remaining is below threshold,
the cycle may begin again with the RF pulse generator module 106 pushing
a notification to the application, as shown in block 1008. If the
battery's power remaining is above threshold the RF pulse generator
module 106 may push a successful battery-change notification to the
application, as shown in block 1020. The application may then communicate
with the RF pulse generator module 106 and displays current system
status, as shown in block 1022.

[0102] FIG. 11 is yet another example flow chart of a process for a
Manufacturer's Representative to program the implanted wireless neural
stimulator. In this implementation, a user wants the Manufacturer's
Representative to set individual parameter programs from a remote
location different than where the user is, for the user to use as needed,
as shown in block 1102. The Manufacturer's Representative can gain access
to the user's set parameter programs through a secure web based service.
The Manufacturer's Representative can securely log into the
manufacturer's web service on a device connected to the Internet, as
shown in block 1104. If the Manufacturer's Representative is registering
the user for the first time in their care they enter in the patient's
basic information, the RF pulse generator's unique ID and the programming
application's unique ID, as shown in block 1106. Once the Manufacturer's
Representative's new or old user is already registered, the
Manufacturer's Representative accesses the specific user's profile, as
shown in block 1108. The Manufacturer's Representative is able to view
the current allotted list of parameter programs for the specific user, as
shown in block 1110. This list may contain previous active and retired
parameter preset programs, as shown in block 1112. The Manufacturer's
Representative is able to activate/deactivate preset parameter programs
by checking the box next to the appropriate row in the table displayed,
as shown in block 1114. The Manufacturer's Representative may then submit
and save the allotted new preset parameter programs, as shown in block
1116. The user's programmer application may receive the new preset
parameter programs at the next sync with the manufacturer's database.

[0103] FIG. 12 is a circuit diagram showing an example of a wireless
neural stimulator, such as stimulator 114. This example contains paired
electrodes, comprising cathode electrode(s) 1208 and anode electrode(s)
1210, as shown. When energized, the charged electrodes create a volume
conduction field of current density within the tissue. In this
implementation, the wireless energy is received through a dipole
antenna(s) 238. At least four diodes are connected together to form a
full wave bridge rectifier 1202 attached to the dipole antenna(s) 238.
Each diode, up to 100 micrometers in length, uses a junction potential to
prevent the flow of negative electrical current, from cathode to anode,
from passing through the device when said current does not exceed the
reverse threshold. For neural stimulation via wireless power, transmitted
through tissue, the natural inefficiency of the lossy material may lead
to a low threshold voltage. In this implementation, a zero biased diode
rectifier results in a low output impedance for the device. A resistor
1204 and a smoothing capacitor 1206 are placed across the output nodes of
the bridge rectifier to discharge the electrodes to the ground of the
bridge anode. The rectification bridge 1202 includes two branches of
diode pairs connecting an anode-to-anode and then cathode to cathode. The
electrodes 1208 and 1210 are connected to the output of the charge
balancing circuit 246.

[0104]FIG. 13 is a circuit diagram of another example of a wireless
neural stimulator, such as stimulator 114. The example shown in FIG. 13
includes multiple electrode control and may employ full closed loop
control. The stimulator includes an electrode array 254 in which the
polarity of the electrodes can be assigned as cathodic or anodic, and for
which the electrodes can be alternatively not powered with any energy.
When energized, the charged electrodes create a volume conduction field
of current density within the tissue. In this implementation, the
wireless energy is received by the device through the dipole antenna(s)
238. The electrode array 254 is controlled through an on-board controller
circuit 242 that sends the appropriate bit information to the electrode
interface 252 in order to set the polarity of each electrode in the
array, as well as power to each individual electrode. The lack of power
to a specific electrode would set that electrode in a functional OFF
position. In another implementation (not shown), the amount of current
sent to each electrode is also controlled through the controller 242. The
controller current, polarity and power state parameter data, shown as the
controller output, is be sent back to the antenna(s) 238 for telemetry
transmission back to the pulse generator module 106. The controller 242
also includes the functionality of current monitoring and sets a bit
register counter so that the status of total current drawn can be sent
back to the pulse generator module 106.

[0105] At least four diodes can be connected together to form a full wave
bridge rectifier 302 attached to the dipole antenna(s) 238. Each diode,
up to 100 micrometers in length, uses a junction potential to prevent the
flow of negative electrical current, from cathode to anode, from passing
through the device when said current does not exceed the reverse
threshold. For neural stimulation via wireless power, transmitted through
tissue, the natural inefficiency of the lossy material may lead to a low
threshold voltage. In this implementation, a zero biased diode rectifier
results in a low output impedance for the device. A resistor 1204 and a
smoothing capacitor 1206 are placed across the output nodes of the bridge
rectifier to discharge the electrodes to the ground of the bridge anode.
The rectification bridge 1202 may include two branches of diode pairs
connecting an anode-to-anode and then cathode to cathode. The electrode
polarity outputs, both cathode 1208 and anode 1210 are connected to the
outputs formed by the bridge connection. Charge balancing circuitry 246
and current limiting circuitry 248 are placed in series with the outputs.

[0106] FIG. 14 is a block diagram showing an example of control functions
1405 and feedback functions 1430 of a wireless implantable neural
stimulator 1400, such as the ones described above or further below. An
example implementation of the implantable neural stimulator 1400 may be
implanted lead module 114, as discussed above in association with FIG. 2.
Control functions 1405 include functions 1410 for polarity switching of
the electrodes and functions 1420 for power-on reset.

[0107] Polarity switching functions 1410 may employ, for example, a
polarity routing switch network to assign polarities to electrodes 254.
The assignment of polarity to an electrode may, for instance, be one of:
a cathode (negative polarity), an anode (positive polarity), or a neutral
(off) polarity. The polarity assignment information for each of the
electrodes 254 may be contained in the input signal received by wireless
implantable neural stimulator 1400 through Rx antenna 238 from RF pulse
generator module 106. Because a programmer module 102 may control RF
pulse generator module 106, the polarity of electrodes 254 may be
controlled remotely by a programmer through programmer module 102, as
shown in FIG. 2.

[0108] Power-on reset functions 1420 may reset the polarity assignment of
each electrode immediately on each power-on event. As will be described
in further detail below, this reset operation may cause RF pulse
generator module 106 to transmit the polarity assignment information to
the wireless implantable neural stimulator 1400. Once the polarity
assignment information is received by the wireless implantable neural
stimulator 1400, the polarity assignment information may be stored in a
register file, or other short term memory component. Thereafter the
polarity assignment information may be used to configure the polarity
assignment of each electrode. If the polarity assignment information
transmitted in response to the reset encodes the same polarity state as
before the power-on event, then the polarity state of each electrode can
be maintained before and after each power-on event.

[0109] Feedback functions 1430 include functions 1440 for monitoring
delivered power to electrodes 254 and functions 1450 for making impedance
diagnosis of electrodes 254. For example, delivered power functions 1440
may provide data encoding the amount of power being delivered from
electrodes 254 to the excitable tissue and tissue impedance diagnostic
functions 1450 may provide data encoding the diagnostic information of
tissue impedance. The tissue impedance is the electrical impedance of the
tissue as seen between negative and positive electrodes when a
stimulation current is being released between negative and positive
electrodes.

[0110] Feedback functions 1430 may additionally include tissue depth
estimate functions 1460 to provide data indicating the overall tissue
depth that the input radio frequency (RF) signal from the pulse generator
module, such as, for example, RF pulse generator module 106, has
penetrated before reaching the implanted antenna, such as, for example,
RX antenna 238, within the wireless implantable neural stimulator 1400,
such as, for example, implanted lead module 114. For instance, the tissue
depth estimate may be provided by comparing the power of the received
input signal to the power of the RF pulse transmitted by the RF pulse
generator 106. The ratio of the power of the received input signal to the
power of the RF pulse transmitted by the RF pulse generator 106 may
indicate an attenuation caused by wave propagation through the tissue.
For example, the second harmonic described below may be received by the
RF pulse generator 106 and used with the power of the input signal sent
by the RF pulse generator to determine the tissue depth. The attenuation
may be used to infer the overall depth of wireless implantable neural
stimulator 1400 underneath the skin.

[0111] The data from blocks 1440, 1450, and 1460 may be transmitted, for
example, through Tx antenna 110 to RF pulse generator 106, as illustrated
in FIGS. 1 and 2.

[0112] As discussed above in association with FIGS. 1, 2, 12, and 13, a
wireless implantable neural stimulator 1400 may utilize rectification
circuitry to convert the input signal (e.g., having a carrier frequency
within a range from about 800 MHz to about 6 GHz) to a direct current
(DC) power to drive the electrodes 254. Some implementations may provide
the capability to regulate the DC power remotely. Some implementations
may further provide different amounts of power to different electrodes,
as discussed in further detail below.

[0113] FIG. 15 is a schematic showing an example of a wireless implantable
neural stimulator 1500 with components to implement control and feedback
functions as discussed above in association with FIG. 14. An RX antenna
1505 receives the input signal. The RX antenna 1505 may be embedded as a
dipole, microstrip, folded dipole or other antenna configuration other
than a coiled configuration, as described above. The input signal has a
carrier frequency in the GHz range and contains electrical energy for
powering the wireless implantable neural stimulator 1500 and for
providing stimulation pulses to electrodes 254. Once received by the
antenna 1505, the input signal is routed to power management circuitry
1510. Power management circuitry 1510 is configured to rectify the input
signal and convert it to a DC power source. For example, the power
management circuitry 1510 may include a diode rectification bridge such
as the diode rectification bridge 1202 illustrated in FIG. 12. The DC
power source provides power to stimulation circuitry 1511 and logic power
circuitry 1513. The rectification may utilize one or more full wave diode
bridge rectifiers within the power management circuitry 1510. In one
implementation, a resistor can be placed across the output nodes of the
bridge rectifier to discharge the electrodes to the ground of the bridge
anode, as illustrated by the shunt register 1204 in FIG. 12.

[0114] FIG. 16 shows an example pulse waveform generated by the MFS sent
to the power management circuitry 1510 of the wireless implantable neural
stimulator 1500. This can be a typical pulse waveform generated by the RF
pulse generator module 106 and then passed on the carrier frequency. The
pulse amplitude is ramped over the pulse width (duration) from a value
ranging from -9 dB to +6 dB. In certain implementations, the ramp start
and end power level can be set to any range from 0 to 60 dB. The gain
control is adjustable and can be an input parameter from RF pulse
generator module 106 to the stimulation power management circuitry 1510.
The pulse width, Pw, can range from 100 to 300 microseconds (μs) in
some implementations, as shown in FIG. 16. In other implementations not
shown, the pulse width can be between about 5 microseconds (5 us) and
about 10 milliseconds (10 ms). The pulse frequency (rate) can range from
about 5 Hz to 120 Hz as shown. In some implementations not shown, the
pulse frequency can be below 5 Hz, and as high as about 10,000 Hz.

[0115] Returning to FIG. 15, based on the received waveform, stimulation
circuitry 1511 creates the stimulation waveform to be sent to the
electrodes 254 to stimulate excitable tissues, as discussed above. In
some implementations, stimulation circuitry 1511 may route the waveform
to pulse-shaping resistor-capacitor (RC) timer 1512 to shape each
travelling pulse waveform. An example RC-timer can be the shunt resistor
1204 and smoothing resistor 1206, as illustrated in FIG. 12 and as
discussed above. The pulse-shaping RC timer 1512 can also be used to, but
is not limited to, inverting the pulse to create a pre-anodic dip or
provide a slow ramping in waveform.

[0116] Once the waveform has been shaped, the cathodic energy--energy
being transmitted over the cathodic branch 1515 of the polarity routing
switch network 1523--is routed through the passive charge balancing
circuitry 1518 to prevent the build-up of noxious chemicals at the
electrodes 254, as discussed above. Cathodic energy is then routed to
input 1, block 1522, of polarity routing switch network 1521. Anodic
energy--energy being transmitted over the anodic branch 1514 of the
polarity routing switch network 1523--is routed to input 2, block 1523,
of polarity routing switch network 1521. Thereafter, the polarity routing
switch network 1521 delivers the stimulation energy in the form of
cathodic energy, anodic energy, or no energy, to the each of the
electrodes 254, depending on the respective polarity assignment, which is
controlled based on a set of bits stored in the register file 1532. The
bits stored in the register file 1532 are output to a selection input
1534 of the polarity routing switch network 1523, which causes input 1 or
input 2 to be routed to the electrodes as appropriate.

[0117] Turning momentarily to FIG. 17, a schematic of an example of a
polarity routing switch network 1700 is shown. As discussed above, the
cathodic (-) energy and the anodic energy are received at input 1 (block
1522) and input 2 (block 1523), respectively. Polarity routing switch
network 1700 has one of its outputs coupled to an electrode of electrodes
254 which can include as few as two electrodes, or as many as sixteen
electrodes. Eight electrodes are shown in this implementation as an
example.

[0118] Polarity routing switch network 1700 is configured to either
individually connect each output to one of input 1 or input 2, or
disconnect the output from either of the inputs. This selects the
polarity for each individual electrode of electrodes 254 as one of:
neutral (off), cathode (negative), or anode (positive). Each output is
coupled to a corresponding three-state switch 1730 for setting the
connection state of the output. Each three-state switch is controlled by
one or more of the bits from the selection input 1750. In some
implementations, selection input 1750 may allocate more than one bits to
each three-state switch. For example, two bits may encode the three-state
information. Thus, the state of each output of polarity routing switch
device 1700 can be controlled by information encoding the bits stored in
the register 1532, which may be set by polarity assignment information
received from the remote RF pulse generator module 106, as described
further below.

[0119] Returning to FIG. 15, power and impedance sensing circuitry may be
used to determine the power delivered to the tissue and the impedance of
the tissue. For example, a sensing resistor 1518 may be placed in serial
connection with the anodic branch 1514. Current sensing circuit 1519
senses the current across the resistor 1518 and voltage sensing circuit
1520 senses the voltage across the resistor. The measured current and
voltage may correspond to the actual current and voltage applied by the
electrodes to the tissue.

[0120] As described below, the measured current and voltage may be
provided as feedback information to RF pulse generator module 106. The
power delivered to the tissue may be determined by integrating the
product of the measured current and voltage over the duration of the
waveform being delivered to electrodes 254. Similarly, the impedance of
the tissue may be determined based on the measured voltage being applied
to the electrodes and the current being applied to the tissue.
Alternative circuitry (not shown) may also be used in lieu of the sensing
resistor 1518, depending on implementation of the feature and whether
both impedance and power feedback are measured individually, or combined.

[0121] The measurements from the current sensing circuitry 1519 and the
voltage sensing circuitry 1520 may be routed to a voltage controlled
oscillator (VCO) 1533 or equivalent circuitry capable of converting from
an analog signal source to a carrier signal for modulation. VCO 1533 can
generate a digital signal with a carrier frequency. The carrier frequency
may vary based on analog measurements such as, for example, a voltage, a
differential of a voltage and a power, etc. VCO 1533 may also use
amplitude modulation or phase shift keying to modulate the feedback
information at the carrier frequency. The VCO or the equivalent circuit
may be generally referred to as an analog controlled carrier modulator.
The modulator may transmit information encoding the sensed current or
voltage back to RF pulse generator 106.

[0122] Antenna 1525 may transmit the modulated signal, for example, in the
GHz frequency range, back to the RF pulse generator module 106. In some
embodiments, antennas 1505 and 1525 may be the same physical antenna. In
other embodiments, antennas 1505 and 1525 may be separate physical
antennas. In the embodiments of separate antennas, antenna 1525 may
operate at a resonance frequency that is higher than the resonance
frequency of antenna 1505 to send stimulation feedback to RF pulse
generator module 106. In some embodiments. antenna 1525 may also operate
at the higher resonance frequency to receive data encoding the polarity
assignment information from RF pulse generator module 106.

[0123] Antenna 1525 may be a telemetry antenna 1525 which may route
received data, such as polarity assignment information, to the
stimulation feedback circuit 1530. The encoded polarity assignment
information may be on a band in the GHz range. The received data may be
demodulated by demodulation circuitry 1531 and then stored in the
register file 1532. The register file 1532 may be a volatile memory.
Register file 1532 may be an 8-channel memory bank that can store, for
example, several bits of data for each channel to be assigned a polarity.
Some embodiments may have no register file, while some embodiments may
have a register file up to 64 bits in size. The information encoded by
these bits may be sent as the polarity selection signal to polarity
routing switch network 1521, as indicated by arrow 1534. The bits may
encode the polarity assignment for each output of the polarity routing
switch network as one of: + (positive), - (negative), or 0 (neutral).
Each output connects to one electrode and the channel setting determines
whether the electrode will be set as an anode (positive), cathode
(negative), or off (neutral).

[0124] Returning to power management circuitry 1510, in some embodiments,
approximately 90% of the energy received is routed to the stimulation
circuitry 1511 and less than 10% of the energy received is routed to the
logic power circuitry 1513. Logic power circuitry 1513 may power the
control components for polarity and telemetry. In some implementations,
the power circuitry 1513, however, does not provide the actual power to
the electrodes for stimulating the tissues. In certain embodiments, the
energy leaving the logic power circuitry 1513 is sent to a capacitor
circuit 1516 to store a certain amount of readily available energy. The
voltage of the stored charge in the capacitor circuit 1516 may be denoted
as Vdc. Subsequently, this stored energy is used to power a power-on
reset circuit 1516 configured to send a reset signal on a power-on event.
If the wireless implantable neural stimulator 1500 loses power for a
certain period of time, for example, in the range from about 1
millisecond to over 10 milliseconds, the contents in the register file
1532 and polarity setting on polarity routing switch network 1521 may be
zeroed. The wireless implantable neural stimulator 1500 may lose power,
for example, when it becomes less aligned with RF pulse generator module
106. Using this stored energy, power-on reset circuit 1540 may provide a
reset signal as indicated by arrow 1517. This reset signal may cause
stimulation feedback circuit 1530 to notify RF pulse generator module 106
of the loss of power. For example, stimulation feedback circuit 1530 may
transmit a telemetry feedback signal to RF pulse generator module 106 as
a status notification of the power outage. This telemetry feedback signal
may be transmitted in response to the reset signal and immediately after
power is back on neural stimulator 1500. RF pulse generator module 106
may then transmit one or more telemetry packets to implantable wireless
neutral stimulator. The telemetry packets contain polarity assignment
information, which may be saved to register file 1532 and may be sent to
polarity routing switch network 1521. Thus, polarity assignment
information in register file 1532 may be recovered from telemetry packets
transmitted by RF pulse generator module 106 and the polarity assignment
for each output of polarity routing switch network 1521 may be updated
accordingly based on the polarity assignment information.

[0125] The telemetry antenna 1525 may transmit the telemetry feedback
signal back to RF pulse generator module 106 at a frequency higher than
the characteristic frequency of an RX antenna 1505. In one
implementation, the telemetry antenna 1525 can have a heightened
resonance frequency that is the second harmonic of the characteristic
frequency of RX antenna 1505. For example, the second harmonic may be
utilized to transmit power feedback information regarding an estimate of
the amount of power being received by the electrodes. The feedback
information may then be used by the RF pulse generator in determining any
adjustment of the power level to be transmitted by the RF pulse generator
106. In a similar manner, the second harmonic energy can be used to
detect the tissue depth. The second harmonic transmission can be detected
by an external antenna, for example, on RF pulse generator module 106
that is tuned to the second harmonic. As a general matter, power
management circuitry 1510 may contain rectifying circuits that are
non-linear device capable of generating harmonic energies from input
signal. Harvesting such harmonic energy for transmitting telemetry
feedback signal could improve the efficiency of wireless implantable
neural stimulator 1500. FIGS. 18A and 18B and the following discussion
demonstrate the feasibility of utilizing the second harmonic to transmit
telemetry signal to RF pulse generator module 106.

[0126] FIGS. 18A and 18BB respectively show an example full-wave rectified
sine wave and the corresponding spectrum. In particular, a full-wave
rectified 915 MHz sine wave is being analyzed. In this example, the
second harmonic of the 915 MHz sine wave is an 1830 MHz output harmonic.
This harmonic wave may be attenuated by the amount of tissue that the
harmonic wave needs to pass through before reaching the external harmonic
receiver antenna. In general, an estimation of the power levels during
the propagation of the harmonic wave can reveal the feasibility of the
approach. The estimation may consider the power of received input signal
at the receiving antenna (e.g., at antenna 1505 and at 915 MHz), the
power of the second harmonic radiated from the rectified 915 MHz
waveform, the amount of attenuation for the second harmonic wave to
propagate through the tissue medium, and an estimation of the coupling
efficiency for the harmonic antenna. The average power transmitted in
Watts can be estimated by Equation 1:

Pt=Pk DuC

Pr=(Pt/Aant)(1-{Γ}2)Lλ2Grη-
/4π) (1)

[0127] Table 1 below tabulates the denotations of each symbol and the
corresponding value used in the estimation.

[0128] In estimating L, the loss due to the attenuation in the tissue,
attenuations from the fundamental (for the forward path to the implanted
lead module 114) and second harmonics (for the reverse path from the
implanted lead module 113) may be considered. The plane wave attenuation
is given by the following equation (2) and Table 2:

[0129] The worst case assumption for coupling of the harmonics wave to the
external receive antenna is that the power radiated at the harmonic
frequency by the implanted telemetry antenna (e.g., telemetry antenna
1625) is completely absorbed by external receive antenna. This worst case
scenario can be modeled by the following equation (3) and Table 3:

[0136] In sum, the reduction of power levels has been estimated to be
about 10 dB utilizing these developed equations. This includes the
attenuation of a 915 MHz plane wave that propagates through tissue depths
from 1 cm to 6 cm. The average received power, Pr, at 915 MHz is 0.356 W.
The power in the second harmonic (1830 MHz) is about -6.16 dB, as
obtained from a SPICE simulation using a full wave rectified 915 MHz sine
wave. The estimate of 10 dB means a reduction of a factor of 10, which is
acceptable for field operations. Thus, the feasibility of utilizing the
second harmonic frequency to transmit the telemetry feedback signal back
to the RF pulse generator module 106 has been demonstrated.

[0137] FIG. 19 is a flow chart illustrating an example of operations of
control and feedback functions of the neural stimulator. The operations
are described with respect to the wireless implantable neural stimulator
1500, although the operations may be performed by other variations of a
wireless implantable neural stimulator, such as the ones described above.

[0138] RF pulse generator module 106 transmits one or more signals
containing electrical energy (1900). RF pulse generator module 106 may
also be known as a microwave field stimulator (MFS) in some
implementations. The signal may be modulated at a microwave frequency
band, for example, from about 800 MHz to about 6 GHz.

[0139] The input signal containing electrical energy is received by RX
antenna 1505 of the neural stimulator 1500 (1910). As discussed above, RX
antenna 1505 may be embedded as a dipole, microstrip, folded dipole or
other antenna configuration other than a coiled configuration.

[0140] The input signal is rectified and demodulated by the power
management circuitry 1510, as shown by block 1911. Some implementations
may provide waveform shaping and, in this case, the rectified and
demodulated signal is passed to pulse shaping RC timer (1912). Charge
balancing may be performed by charge balancing circuit 1518 to provide a
charged balanced waveform (1913). Thereafter, the shaped and charge
balanced pulses are routed to electrodes 254 (1920), which deliver the
stimulation to the excitable tissue (1921).

[0141] In the meantime, the current and voltage being delivered to the
tissue is measured using the current sensor 1519 and voltage sensor 1520
(1914). These measurements are modulated and amplified (1915) and
transmitted to the RF pulse generator module 106 from telemetry antenna
1525 (1916). In some embodiments, the telemetry antenna 1525 and RX
antenna 1505 may utilize the same physical antenna embedded within the
neural stimulator 1500. The RF pulse generator module 106 may use the
measured current and voltage to determine the power delivered to the
tissue, as well as the impedance of the tissue.

[0142] For example, the RF pulse generator module 106 may store the
received feedback information such as the information encoding the
current and voltage. The feedback information may be stored, for
instance, as a present value in a hardware memory on RF pulse generator
module 106. Based on the feedback information, RF pulse generator module
106 may calculate the impedance value of the tissue based on the current
and voltage delivered to the tissue.

[0143] In addition, RF pulse generator module 106 may calculate the power
delivered to the tissue based on the stored current and voltage (1950).
The RF pulse generator module 106 can then determine whether power level
should be adjusted by comparing the calculated power to the desired power
stored, for example, in a lookup table stored on the RF pulse generator
module 106 (1917). For example, the look-up table may tabulate the
optimal amount of power that should be delivered to the tissue for the
position of the receive antenna 1505 on neural stimulator 1500 relative
to the position of the transmit antenna on RF pulse generator module 106.
This relative position may be determined based on the feedback
information. The power measurements in the feedback information may then
be correlated to the optimal value to determine if a power level
adjustment should be made to increase or decrease the amplitude of
stimulation of the delivered power to the electrodes. The power level
adjustment information may then enable the RF pulse generator module 106
to adjust parameters of transmission so that the adjusted power is
provided to the RX antenna 1505.

[0144] In addition to the input signal containing electrical energy for
stimulation, the RF pulse generator module 106 may send an input signal
that contains telemetry data such as polarity assignment information
(1930). For instance, upon power on, the RF pulse generator module 106
may transmit data encoding the last electrode polarity settings for each
electrode before RF pulse generator module 106 was powered off. This data
may be sent to telemetry antenna 1525 as a digital data stream embedded
on the carrier waveform. In some implementations, the data stream may
include telemetry packets. The telemetry packets are received from the RF
pulse generator module 106 and subsequently demodulated (1931) by
demodulation circuit 1531. The polarity setting information in the
telemetry packets is stored in the register file 1532 (1932). The
polarity of each electrode of electrodes 254 is programmed according to
the polarity setting information stored in the register file 1532 (1933).
For example, the polarity of each electrode may be set as one of: anode
(positive), cathode (negative), or neutral (off).

[0145] As discussed above, upon a power-on reset, the polarity setting
information is resent from the RF pulse generator module 106 to be stored
in the register file 1532 (1932). This is indicated by the arrow 1932 to
1916. The information of polarity setting stored in the register file
1532 may then be used to program the polarity of each electrode of
electrodes 254 (1933). The feature allows for re-programming of a passive
device remotely from the RF pulse generator module 106 at the start of
each powered session, thus obviating the need of maintaining CMOS memory
within the neural stimulator 1500.

[0146] A number of implementations have been described. Nevertheless, it
will be understood that various modifications may be made. Accordingly,
other implementations are within the scope of the following claims.

Patent applications by Chad Andresen, Chandler, AZ US

Patent applications by Laura Tyler Perryman, Scottsdale, AZ US

Patent applications by Patrick Larson, Scottsdale, AZ US

Patent applications by STIMWAVE TECHNOLOGIES INCORPORATED

Patent applications in class Telemetry or communications circuits

Patent applications in all subclasses Telemetry or communications circuits